Surface polysaccharides of Gram-negative pathogens, capsules or lipopolysaccharides (LPS), are essential virulence factors and protective antigens. The immunogenicity of polysaccharides is enhanced by covalent binding to carrier proteins. To serve as vaccines, LPSs must be detoxified and their O-specific polysaccharides (O-SP) isolated and conjugated to proteins. Bacterial toxins or toxoids and viral capsid proteins may be protective antigens and may serve as carrier proteins. The capsular polysaccharide of Salmonella typhi (Vi) is a licensed vaccine but with limited efficacy in children less than 5 years old. To provide a vaccine for younger children, Vi was conjugated to recombinant exoprotein A of Pseudomonas aeruginosa (rEPA). An efficacy of 89% at 47 months was shown in 2-to-5-year olds injected with Vi-rEPA. Safety and immunogenicity of Vi-rEPA were evaluated in 301 Vietnamese infants administered concurrently with their routine immunizations at 2, 4, 6 and 12 months. Controls received Hib-TT +DTP or DTP. No serious adverse events occurred in any group. The post immunization GM IgG anti-Vi level in the Vi-rEPA group was significantly higher than in the control groups. There was no difference in the level of IgG anti-DT among the groups. Antibody levels to TT and PT are being assayed. Fruit pectin is composed of polygalacturonic acid, structurally identical to the backbone of Vi. Pectin can be O-acetylated chemically at the C2 and C3 positions (OacPec) to mimic Vi antigenically. A phase I study of OacPec-rEPA injected once into 25 healthy adult volunteers showed it to be safe. Six weeks after injection, a >4 fold rise in IgG anti-Vi was found in 60% of the volunteers. No rise in IgG anti-pectin was found. Salmonella paratyphi A (SPA) is the second most common cause of enteric fever in developing countries transmited is through ingestion of food or drink contaminated by infected persons. The presence of chronic carriers of SPA is not established. In collaboration with the Guangxi Center for Disease Control and Prevention, we investigated a large outbreak in a residential middle school. A chronic carrier, most likely the source of the outbreak, was identified by having an exceptionally high level of serum IgG anti-LPS and a positive rectal swab. An expanded survey, using the same methods, was initiated. Vibrio cholerae O1 remains a major health problem in the Indian subcontinent and in Africa. Field studies showed that serum vibriocidal activity is directed toward its LPS. In a phase 1 trial, O-SP conjugates elicited IgG anti-LPS with vibriocidal activity. To enhance immunogenicity, a conjugation scheme was devised employing derivatization of both the carrier protein and the O-SP with heterobifunctional reagents. Conjugates of a V. cholerae O-SP hexamer we synthetized and the improved O-SP conjugates induced higher vibriocidal activities than the conjugate used in the Phase I study. Rotavirus is the most common cause of infantile diarrhea worldwide. Two rotavirus vaccines are licensed, a live attenuated and a reassortant, both orally administered. We are designing a parenteral vaccine based on capsid proteins. Recombinant capsid proteins with truncated C or N termini were expressed in E. coli. Mice injected with the recombinant proteins developed neutralizing antibodies. Conjugation of the recombinant proteins to polysaccharide increased their solubility